This invention relates to a molecular sieve type gas separation apparatus for separating product gas from a gas stream. A prior art molecular sieve type gas separation apparatus is described in prior European specification 0129304 and comprises in that example, three sieve beds which each contain a material which adsorbs nitrogen and other gases from a gas supply whilst permitting product gas, containing oxygen in that example, to pass from the apparatus and be used in an oxygen breathing system for example for an air crew in an aircraft.
In prior specification EP0129304, there is described an apparatus in which the operation of each sieve bed is cycled. In each cycle, each sieve bed is subjected to an adsorption charge phase in which nitrogen primarily, is adsorbed by the sieve bed, and a desorption regeneration phase in which the nitrogen, which is non product gas, is desorbed from the sieve bed. This is achieved, as is well known in the art, by introducing the gas supply to the sieve bed at a first pressure during the adsorption charge phase, and during the desorption regeneration phase, the bed is subjected to a second pressure, lower than the first.
For an air crew, it is necessary to provide increasing concentrations of oxygen in the product gas for breathing, with increasing altitude. In prior specification EP0129304, the concentration of oxygen in the product gas is varied by adjusting the overall cycle time for the or each sieve bed whilst the relative durations of the adsorption charge and desorption regeneration phases are retained substantially constant.
Although the prior art apparatus works satisfactorily in practice, there is a requirement to make the apparatus more effective.
Experiments into the operation of sieve beds have shown how the adsorbent material is better cleansed of the adsorbed gas by increasing the duration of the desorption regeneration phase in each cycle.
According to a first aspect of the invention we provide a molecular sieve type gas separation apparatus for separating product gas from a gas supply, the apparatus comprising at least one sieve bed which during an adsorption charge phase of a cycle, adsorbs non product gas in the gas supply, and in a desorption regeneration phase of the cycle desorbs the previously adsorbed non product gas, the apparatus further comprising outlet means to which product gas is delivered during the adsorption charge phase, and outlet means for non product gas during the desorption regeneration phase, sensor means responsive to the concentration of a constituent in the product gas, and control means to switch the operation of the sieve bed between the adsorption charge and desorption regeneration phases, characterised in that the control means is adapted to adjust the duration of the adsorption charge phase relative to the duration of the desorption regeneration phase in each cycle.
Thus the applicants have found that by adjusting the duration of adsorption charge phase relative to the duration of desorption regeneration phase in each cycle of the sieve bed (or beds) improved control over the concentration of desired constituent in the product gas can be achieve.
Whereas in prior European specification 0129304 the cycle time is varied during an operation period, whilst the relative durations of adsorption charge and desorption regeneration phases are maintained generally constant, in the apparatus of the invention the overall cycle time for the or each sieve bed may be maintained generally constant during an operation period, whilst the relative durations of adsorption charge and desorption regeneration phases in each cycle are adjusted.
The invention provides various advantages. Particularly, the deviation of concentration of desired constituent in the product gas from a desired level can be greatly reduced in general and thus the risk of producing product gas with a lower or higher than acceptable concentration of desired constituent therein, is greatly reduced.
Furthermore, particularly where low concentrations of desired constituent in the product gas are desired, the duration of the desorption regeneration phase may be reduced, which means that the overall amount of gas supply which needs to be fed to the apparatus can be reduced, resulting in higher efficiency.
In a preferred arrangement, the apparatus includes a plurality of sieve beds which may be operated by the control means out of phase with each other such that in use, at least one of the beds is always operating in an adsorption charge phase and is thus producing product gas.
Preferably therefore, where there are N sieve beds switched between adsorption charge and desorption regeneration phases in common cycle times but, out of phase with one another, the apparatus may be operated such that the proportion of duration of adsorption charge phase to total cycle time in each bed is always greater than 1:N.
To achieve switching between the adsorption charge and desorption regeneration phases, each bed of the apparatus may have associated valves which are openable and closable by the control means to permit the introduction of the gas supply and delivery of the product gas during the adsorption charge phase, and venting of the non product gas during the desorption regeneration phase.
The apparatus may comprise a container containing the or each sieve bed, and although each valve may be a simple open/closed valve which permits appropriate gas to enter or leave the container, if desired at least one of the valves may be an active valve whereby the extent of opening and closing of the valve to permit one of the gas supply or product gas or non product gas to enter or leave the container is adjustable and/or the rate at which the valve may be switched between open and closed positions may be variable.
At the end of the desorption regeneration phase of the or each bed, prior to the next adsorption charge phase, if desired product gas may be introduced into the apparatus to purge remaining non product gas from the container.
Between at least some of the adsorption charge and desorption regeneration phases, there may be a pause phase in which no or minimal gas supply or product gas or non product gas is permitted to enter or leave the container.
Between at least some of the adsorption charge and desorption regeneration phases, there may be an overlap phase when at least two of the feeding of gas supply to the bed, product gas from the bed and non product gas from the bed, may occur simultaneously.
The invention is particularly but not exclusively applicable to a so called oxygen concentrator in which case the desired monitored constituent in the product gas may be oxygen and the or each sieve bed may contain sieve bed material capable of adsorbing nitrogen gas, so that the produced gas may be used for breathing, e.g. by an air crew member in an aircraft. Thus the control means may operate the apparatus so as to achieve an oxygen concentration in the product gas close to a target concentration, which may vary with altitude.
According to a second aspect of the invention we provide an aircraft having a gas separation apparatus according to the first aspect of the invention.
According to a third aspect of the invention we provide a method of operating an apparatus according to the first aspect of the invention, comprising the steps of sensing the concentration of a constituent in the product gas, and adjusting the duration of adsorption charge phase relative to the duration of desorption regeneration phase in each cycle for the or each sieve bed so as to achieve a concentration of constituent in the product gas within predetermined limits.